Repair techniques for lacerated or severed tendons and ligaments (“connective tissues” or “cords”) vary widely depending on the nature of the injury and the particular cord affected. There are large differences in the extent to which access can be obtained in the at least obtrusive manner, in the amount of cord excursion, in the surrounding environment, in the stresses to which different cords are normally subjected, and in the healing characteristics of different cords. In addition, often there is no consensus of the overall best way to repair a given cord. Examples of often injured cords having different accepted repair techniques are flexor tendons of the hand and the anterior cruciate ligament (ACL) of the knee.
For example, repair of a long flexor tendon that has been severed is typically achieved by suturing the severed tendon ends face-to-face. Historically, the joints across which the tendon acts were immobilized for from three to eight weeks to protect the tendon while it healed, because a freshly sutured tendon can withstand only a fraction of the tensile force to which a healthy tendon is subjected during normal use. Immobilization results in scarring and adhesion formation along the length of the tendon. Range of motion is adversely affected, particularly in the case of flexor tendons which normally glide smoothly through and over the unique system of tendon tunnels and pulleys of the hand. Nevertheless, it was thought that fibroblastic ingrowth was required in order for the tendon to heal, such that immobilization and the resulting decreased range of motion were considered necessary evils in order for effective healing to take place.
More recently, it has been discovered that flexor tendons have an intrinsic capacity to heal and that limited motion will actually expedite healing. The affected joints are most often partially immobilized to prevent inadvertent application of excess force.
In the case of an anterior cruciate ligament (connecting the bottom of the femur and the top of the tibia) the stresses resulting from applied forces are much greater, there is less interaction with surrounding tissue and bone, the excursion of the cord is less, and the healing tendencies are vastly different. Despite numerous studies, there still is no universally accepted repair procedure, and prevailing procedures are difficult and intricate. The current “standard of care” remains the reconstruction of the ACL using a bone-tendon-bone autograft (i.e., harvested from the patient). However, there are multiple problems with bone-tendon-bone grafting. (1) The intact ACL possesses important mechanoreceptive and proprioceptive capabilities. Graft reconstruction sacrifices these capabilities. (2) Autografting involves considerable donor site morbidity. (3) To avoid donor site morbidity, occasionally a cadaveric graft is used. This carries the risk of disease transmission.
Thus, it is highly advantageous to have an in vivo and ex vivo methods of inducing and/or enhancing the repair of damaged connective tissue.